Glorious

I try to discourage people from using systems older than Sandy Bridge as file servers. The problem is power consumption. File servers are meant to stay on 24/7. Intel didn't take reducing idle power consumption seriously until Sandy Bridge. Consequently a lot of these older systems will idle at around 100 Watts, vs about 20-25 Watts for a modern system, 5-15 Watts for a proper NAS (and most of that is the hard drives). The only review I could find for an E6400 system was in German, and stated idle power consumption was 123 Watts.https://www.pcwelt.de/produkte/Intel-Core-2-Duo-E6400-195337.html

If you pay the U.S. average electricity price of 11.5 cents/kWh, each Watt for a device left on 24/7 translates into almost exactly $1 per year. So the difference between a 100 Watt old system and a 15 Watt NAS is $85 per year. In a few years, the extra electricity cost ends up exceeding the cost of a NAS. So it's actually cheaper to just buy a NAS rather than to try to recycle such an old system. Both Synology and QNAP make very good dedicated NASes. But depending on your requirements you might even be able to just get an external HDD and plug it into the USB port on your router (the newer routers with USB 3 ports can usually achieve 50+ MB/s throughput).

The WD Red drives are only necessary if you're buying WD and using the drives in RAID. WD intentionally removed a feature to make RAID operation more reliable from all their drives except the red drives. All other drive manufacturers retain this feature (TLER) in all their drives. WD is basically forcing you to pay extra for a feature all other drive manufacturers include through their entire lineup.

If you do elect to use your old system rather than buy a NAS and you don't have an extra Windows license, look into FreeNAS. It's based on ZFS which has a lot of features and protections compared to NTFS, HFS, ext4, FAT32, etc. The learning curve is a bit steep though. Don't be put off by the 8 GB RAM requirement. That's really only necessary if you use deduplication, which I definitely recommend you NOT use unless you absolutely need it (it's a CPU and RAM hog which slows down file write speeds by a huge amount). I've got it FreeNAS with deduplication turned off running with 4 GB without problems, and looking at the memory logs it would probably do just fine with 2 GB.